Method for depositing highly erosive and abrasive wear resistant composite coating system on a substrate
This is a division of application Ser. No. 07/153,738, filed Feb.  8, 1988, now U.S. Pat. No. 4,855,188. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The invention relates to highly erosive and abrasive wear resistant composite coating. More particularly, the invention relates to an improved highly erosive and abrasive wear resistant coating comprising a composite coating system of an intermediate layer of substantially pure tungsten and an outer two phase layer of a mixture of tungsten and tungsten carbide. 
     B. Background Art 
     High hardness materials are widely used as coatings on various type of mechanical components and cutting tools. Such coatings impart erosion and abrasion wear resistance and thus increase the erosive and abrasive wear life of objects that have been coated. The high hardness materials can also be used to produce free standing objects which are erosive and abrasive wear resistant. 
     Chemical vapor deposition processes can be used to produce highly erosive and abrasive wear resistant hard coatings and free standing objects. In a typical chemical vapor deposition (CVD) process, the substrate to be coated is heated in a suitable chamber and then a gaseous reactant mixture is introduced into the chamber. The gaseous reactant mixture reacts at the surface of the substrate to form a coherent and adherent layer of the desired coating. By varying the gaseous reactant mixture and the CVD process parameters, various types of deposited coatings can be produced. 
     In U.S. patent application Ser. No. 092,809, filed 3 Sept. 1987, now U.S. Pat. No. 4,874,642, issued 17 Oct. 1989, extremely hard, fine grained, non-columnar, substantially lamellar tungsten/carbon alloys are described which are produced by chemical vapor deposition. The described alloys consist primarily of a mixture of a substantially pure tungsten phase and at least one carbide phase wherein the carbide phase consists of W 2  C or W 3  C or a mixture of W 2  C and W 3  C. The disclosed tungsten/carbon alloys are free of columnar grains and consist essentially of extremely fine, equiaxial crystals. 
     It has been found that the tungsten/carbon alloys such as those described in the aforementioned U.S. patent application, when deposited upon certain types of substrates, exhibit a very fine micro-crack system throughout the deposit. On many types of substrates and under many types of erosive and abrasive wear conditions, preferential attack occurs at the cracks, resulting in poor erosion and abrasion wear resistance for such coatings. 
     The use of an intermediate layer of substantially pure tungsten followed by a tungsten carbide coating is described in the prior art. For example, U.S. Pat. No. 3,389,977 discloses a method of depositing substantially pure tungsten carbide in the form W 2  C wherein the adherence of W 2  C to a steel substrate is improved by first cleaning the surface and then depositing a thin film of tungsten. The thin film of tungsten is deposited at or above 600° C., making the use of the deposition process unsuitable for providing erosive and abrasive wear resistance coating on various carbon steels, stainless steels, nickel and titanium alloys without severely degrading their mechanical properties. Additionally, pure W 2  C deposited according to this patent consists of columnar grains as opposed to non-columnar grains described in the present patent application. Other instances of the use of very thin tungsten intermediate layers, often as a diffusion layer, are reported in other prior art in order to improve adhesion of tungsten carbide on a substrate. However, there is no report in the prior art of the effect of a tungsten interlayer on coating properties of the final coating system nor has the effect of such a tungsten intermediate layer on the reduction or elimination of cracks in the outer coating been reported. 
     SUMMARY OF THE INVENTION 
     Very generally, the highly erosive and abrasive wear resistant composite coating system of the invention comprises an intermediate layer of tungsten and an outer layer of tungsten/carbon alloy coating. The intermediate layer of tungsten is of sufficient thickness to confer substantial erosive and abrasive wear resistance characteristics to the composite coating system. The outer tungsten/carbon alloy layer is comprised of a mixture of tungsten and tungsten carbide, with the tungsten carbide phase comprising of W 2  C, W 3  C or a mixture of both. The ratio of the thickness of the intermediate or inner layer to the thickness of the outer layer is at least above 0.3 in the cases of W+W 3  C, W+W 2  C+W 3  C and W+W 2  C coatings. Preferably the ratio of the thickness of the inner layer to the thickness of the outer layer to get optimum erosion and abrasion wear performance is at least 0.35 in the case of mixtures of tungsten and W 2  C in the outer layer, 0.60 in the case of mixtures of tungsten and W 3  C in the outer layer and, 0.35 in the case of mixtures of tungsten and W 2  C and W 3  C in the outer layer. 
     The tungsten/carbon alloys or coatings consisting of a mixture of tungsten and tungsten carbide, with the tungsten carbide phase comprising W 2  C, W 3  C or mixtures of both are defined herein as tungsten/tungsten carbide to simplify the description. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is the photomicrograph at 1,000 magnification of the tungsten coating on AM-350 stainless steel showing a rough surface finish but the absence of cracks. 
     FIG. 2 is the cross-sectional view at 3,000 magnification of the tungsten coating on AM-350 stainless steel showing columnar growth structure. 
     FIG. 3 is the schematic of the unetched cross-sectional view at 2,000 magnification of the composite coating system on AM-350 stainless steel substrate constructed in accordance with the invention. 
     FIG. 4 is the schematic of the cross-sectional view at 2,000 magnification of the composite coating system on AM-350 stainless steel etched with the murakami solution constructed in accordance with the invention. 
     FIG. 5 is the photomicrograph of the W+W 3  C coating without the tungsten interlayer at 1,000 magnification on AM-350 stainless steel showing a network of interconnected cracks. 
     FIG. 6 is the photomicrograph of the W+W 3  C coating with the tungsten interlayer at 1,000 magnification on AM-350 stainless steel showing a few interconnected cracks. 
     FIG. 7 is the photomicrograph of the W+W 3  C coating with the tungsten interlayer at 1,000 magnification on AM-350 stainless steel showing the absence of cracks. 
     FIG. 8 is the photomicrograph of the surface of W+W 3  C coating without the tungsten interlayer on AM-350 stainless steel at 100 magnification, scratched with a diamond stylus and showing significant loss of the coating in the 30-40 Newton load range. 
     FIG. 9 is the photomicrograph of the surface of W+W 3  C coating with the tungsten interlayer on AM-350 stainless steel at 100 magnification, scratched with a diamond stylus and showing significantly reduced loss of the coating in the 30-40 Newton load range. 
     FIG. 10 is the photomicrograph of the W+W 2  C+W 3  C coating without the tungsten interlayer on AM-350 stainless steel at 1,000 magnification showing a network of cracks. 
     FIG. 11 is the photomicrograph of the W+W 2  C+W 3  C coating with the tungsten interlayer on AM-350 stainless steel at 1,000 magnification showing the absence of cracks. 
     FIG. 12 is the photomicrograph of the W+W 2  C coating with the tungsten interlayer on AM-350 stainless steel at 1,000 magnification showing the presence of a fine crack. 
     FIG. 13 is the photomicrograph of the W+W 2  C coating with the tungsten interlayer on AM-350 stainless steel at 1,000 magnification showing the absence of cracks. 
     FIG. 14 is the graph illustrating the relationship between the erosion rate and the ratio of the tungsten to the tungsten/carbon alloy coating thicknesses on AM-350 stainless steel, and 
     FIG. 15 is the graph illustrating the relationship between the tungsten/carbon alloy coating thickness and the ratio of the thicknesses of the tungsten layer to the tungsten/carbon alloy layer on AM-350 stainless steel. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a preferred form of the invention, the intermediate layer of tungsten is produced by chemical vapor deposition under sub-atmospheric to slightly atmospheric pressure, i.e., within the range of about 1 Torr. to about 1,000 Torr., at a temperature of about 300° to about 650° C., using a mixture of tungsten halide such as WF 6 , hydrogen, and an inert gas such as argon. The intermediate layer is of a sufficient thickness to confer substantial erosive and abrasive wear resistance characteristics on the composite system. The specific thickness necessary to do this for various composite coating systems will become readily apparent to those skilled in the art from the teaching of this specification, particularly in connection with the examples set forth below. The intermediate layer of tungsten should be at least about two microns thick and, for most systems, will be greater than about three microns. 
     Following deposition of the intermediate layer of substantially pure tungsten, an outer layer of tungsten/tungsten carbide is deposited under sub-atmospheric to slightly atmospheric pressure, i.e., within the range of about 1 Torr. to about 1,000 Torr., at temperatures in the range of about 300° to about 650° C. This outer layer may be either a two phase layer comprising tungsten and W 2  C or tungsten and W 3  C. Alternatively, this outer layer may be a three phase layer comprising tungsten, W 2  C and W 3  C. The relative proportions of the tungsten, W 2  C, and W 3  C may be selected in accordance with the particular properties desired in the final composite coating system. In order to achieve such proportions, the tungsten/tungsten carbide deposit is applied utilizing a flow of tungsten halide such as WF 6 , argon, hydrogen and an oxygen containing hydrocarbon such as dimethylether (DME). By controlling the temperature, W/C atomic ratio in the feed gas and the ratio of hydrogen to WF 6  during the deposition reaction, the particular desired chemical composition of the tungsten/tungsten carbide layer may be obtained. Details of the foregoing described process may be found in applicant&#39;s U.S. patent application Ser. No. 92,809 now U.S. Pat. No. 4,874,642, issued 17 Oct. 1989. 
     In accordance with the present invention, it has been found that the ratio of the thickness of the inner tungsten layer to the thickness of the outer multi-phase tungsten/tungsten carbide layer has a profound affect on the erosive and abrasive wear resistance properties of the resulting composite coating system. Although the reasons for this improvement in the erosive and abrasive wear resistance are yet not fully understood, it is believed that the use of the tungsten intermediate layer together with the specified ratios set forth below refine the micro-crack structure in the outer deposit so that, even though preferential attack along the crack system occurs, the rate of attack is greatly attenuated. Moreover, as set out below, under certain conditions a crack free outer layer may be achieved. 
     More specifically, the ratio of the thickness of the inner tungsten layer to the thickness of the outer tungsten/tungsten carbide layer, in accordance with the composite coating system of the invention, is at least above 0.30 with the W+W 3  C, W+W 2  C+W 3  C and W+W 2  C coatings. More specifically, to obtain optimum erosion and abrasion wear performance the thickness ratio is at least: 0.35 in the case of mixtures of tungsten and W 2  C in the outer layer, 0.60 in the case of mixtures of tungsten and W 3  C in the outer layer, and 0.35 in the case of mixtures of tungsten and W 2  C and W 3  C in the outer layer. Using these minimum ratios, superior erosive and abrasive wear resistance can be achieved. Moreover, by using the ratios set forth above, under certain conditions, completely crack free outer layers may be achieved. 
     The inner tungsten layer is substantially columnar in its grain structure with the longer dimension of the grains extending generally perpendicular to the substrate surface. On the other hand, the grain structure of the tungsten/tungsten carbide outer layer is very fine-grained, equiaxed, non-columnar, and substantially lamellar typically of the order of one micron or less in size. Such structures may be readily achieved using the method described in the aforementioned U.S. patent application. 
     The present composite coating system of the invention can be deposited on a number of ferrous metals and alloys such as cast irons, carbon steels, stainless steels and high speed steels, non-ferrous metals and alloys such as copper, nickel, platinum, rhodium, titanium, aluminum, silver, gold, niobium, molybdenum, cobalt, tungsten, rhenium, copper alloys and nickel alloys such as inconel and monel, titanium alloys such as Ti/Al/V, Ti/Al/Sn, Ti/Al/Mo/V, Ti/Al/Sn/Zr/Mo, Ti/Al/V/Cr, Ti/Mo/V/Fe/Al, Ti/Al/V/Cr/Mo/Z and Ti/Al/V/Sn alloys, non-metals such as graphite, carbides such as cemented carbide, and ceramics such as silicon carbide, silicon nitride, alumina, etc. In depositing the composite coating system on reactive substrate materials, such as cast irons, carbon steels, stainless steels, high speed steels, and nickel and monel alloys, it is preferred to coat the substrate first with a more noble material such as nickel, cobalt, copper, silver, gold, platinum, palladium or iridium, by electrochemical or electroless techniques or by a physical vapor deposition technique such as sputtering. In depositing the composite coating system on reactive titanium or titanium alloys, it is also preferred to coat the substrate first with a more noble material described above by electroless technique or by physical vapor deposition technique such as sputtering. It is also preferred to coat the substrate first with a thin layer of a more noble material described above by electroless technique followed by another thin layer of a more noble material by electrochemical or physical vapor deposition technique. It is also preferred to clean the surface of the titanium or titanium alloy substrate first and heat treat the noble metal deposit after depositing on the substrate. The deposition of noble metal and subsequent heat treatment steps on titanium or titanium alloys are described in detail in U.S. patent application Ser. No. 139,891, filed 31 Dec. 1987 now U.S. Pat. No. 4,902,535, issued 20 Feb. 1990. It is also preferred that upper limit of the deposition temperature be about 525° C. when depositing the present composite coating system on titanium and titanium alloys to minimize the degradation of the mechanical properties. No deposition of the noble material, however, is required for coating non-reactive materials such as copper, nickel, cobalt, silver, gold, platinum, rhodium, niobium, molybdenum, tungsten, rhenium, graphite, carbides and ceramics. Free standing parts of the composite coating of the present invention can be made by depositing it on the substrates such as copper, nickel, cobalt, silver, gold, molybdenum, rhenium, and graphite and then removing these substrates by grinding and chemical or electrochemical etching. 
     To further illustrate the present invention, the following data are set forth with respect to a number of coating systems. 
     A number of ferrous and non-ferrous metals and alloys were used as substrates in the following coating experiments. Specimens of AM-350 and SS-422 stainless steels Inconel and IN-718, a well known nickel alloy, were electroplated with 2 to 5 μm thick nickel before coating them with tungsten and tungsten/tungsten carbide to protect them from the attack of hot and corrosive HF acid gas produced as a by-product in the CVD process. Specimens made of titanium alloy such as Ti/6Al/4V, on the other hand, were plated with 2 to 5 μm thick nickel using an electroless technique described in detail in U.S. patent application Ser. No. 139,891, filed 31 Dec. 1987 now U.S. Pat. No. 4,902,535, issued 20 Feb. 1990 before coating them with tungsten and tungsten/tungsten carbide. 
     EROSION TEST PROCEDURE 
     The erosion resistance performance of the uncoated and the coated specimens was determined using a miniature sandblast unit; the test parameters are summarized in Table 1. The fine alumina powder, which was used as the erosive material, provided a very harsh erosion environment, as compared to sand erosion; consequently, an accelerated test procedure could be used. Two essentially equivalent techniques were used to evaluate the erosion resistance of the specimens. The first technique involved measuring the time it took for the erosive material to penetrate the tungsten/tungsten carbide coating, which is called the breakthrough time. Penetration of the tungsten/tungsten carbide coating was visibly apparent as a color change at the center of the erosion pit; the fact that this color change corresponded to the initial penetration of the tungsten/tungsten carbide coating was confirmed by microscopic examination of erosion pits in cross section. The second technique involved measuring the weight of a specimen that was lost during an erosion test for a given time; this time was always less than the breakthrough time so that only the weight loss of the coating was measured. The erosion rate was then calculated as the time required to penetrate the coating on a per mil basis or as the average weight loss for a 30 second erosion test, respectively. 
     EXAMPLES 
     EXAMPLE 1 
     Uncoated specimens of AM-350 stainless steel and Ti/6Al/4V were eroded with alumina for two minutes (120 seconds). The depth of the crater was measured to calculate the erosion rate. The calculated erosion rate was 60 and 50 seconds/mil for AM-350 and Ti/6Al/4V specimens, respectively. 
     EXAMPLE FOR TUNGSTEN COATING 
     EXAMPLE 2 
     A number of AM-350, Ti/6Al/4V and IN-718 specimens (0.095 inch×1 inch×2 inch) were placed in an inductively heated graphite furnace inside a gas-tight quartz envelope. The specimens were heated to 460° C. in the presence of flowing argon gas and at the temperature a gaseous mixture of 300 cc/min of WF 6 , 3,000 cc/min of hydrogen, and 4,000 cc/min of argon was passed into the furnace over the specimens. The total pressure within the system was maintained at 40 Torr. The deposition was conducted for 15 minutes; thereafter, the flow of the reactive gases was stopped and the specimens were cooled. 
     The specimens were found to be coated with a dull, adherent, coherent, and uniform coating. The coating thickness on stainless steel specimens was ˜12 μm on each side (see Table 2). The coating had a rough surface finish and was free of cracks, as shown in the FIG. 1. The coating consisted of columnar growth structure, as shown in the FIG. 2. X-ray diffraction analysis showed the presence of only tungsten in the coating. It had a hardness of 455 Vickers, as shown in Table 3. The coating showed very poor erosion performance; time required to penetrate the coating was only 3 seconds, resulting in a erosion rate of 6 seconds/mil. This, therefore, indicated that CVD tungsten could not be used to provide erosion protection. 
     
                       TABLE 1                                                     
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Erosion Test Procedure                                                    
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Nozzle Diameter   0.018 inch                                              
Stand off Distance                                                        
                  0.6 inch                                                
Erosion Media     Fine Alumina Powder                                     
                  (50 μm Average Particle                              
                  Size)                                                   
Supply Pressure   32 psig                                                 
Flow Rate of Erosion                                                      
                  1.6 g/min                                               
Media                                                                     
Erosion Test Standard                                                     
                  Breakthrough Time and                                   
                  Weight Loss                                             
______________________________________                                    
 
    
     
                                           TABLE 2                                 
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               Example 2                                                  
                     Example 3 Example 4A Example 4B                      
                                                    Example               
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                                                    4C                    
Experiment No.   50    36        37         33        42                  
Substrate      AM-350                                                     
                     AM-350    AM-350                                     
                                    Ti/6A1/4V                             
                                          AM-350    AM-350                
                                                         Ti-6A1/4V        
Temperature, °C.                                                   
                 460   460       460        465       460                 
Pressure, Torr   40    40        40         40        40                  
W Coating Conditions                                                      
H.sub.2, SCCM  3,000 --        3,000      3,000     3,000                 
Ar, SCCM       4,000 --        --         --        --                    
WF.sub.6, SCCM   300 --          300        300       150                 
Time, Min.       15  --           5          5        30                  
Tungsten/Tungsten Carbide                                                 
Conditions                                                                
H.sub.2, SCCM  --    3,000     3,000      3,000     3,000                 
Ar, SCCM       --    --        --         --        --                    
WF.sub.6, SCCM --      300       300        300       300                 
DME, SCCM      --      40        40         40        40                  
Time, Min.     --      40        55         35        20                  
Coating Thickness, μm                                                  
Tungsten       12.0  --        3.0  2.0   3.0       5.0  3.0              
Tungsten/Tungsten Carbide                                                 
               --    22.0      27.0 28.0  25.0      13.0 14.0             
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
               --     0.0      0.11 0.07  0.12      0.38 0.21             
Thickness                                                                 
Surface Topography                                                        
               Rough,                                                     
                     Smooth,   Smooth, Many Fine                          
                                          Smooth,   Smooth, Many Fine     
               No Cracks                                                  
                     Many Fine Interconnected                             
                                          Many Fine Interconnected        
                     Interconnected                                       
                               Cracks     Interconnected                  
                                                    Cracks                
                     Cracks               Cracks                          
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              Example 4D                                                  
                     Example 4E                                           
                             Example 4F Example 4G                        
                                                Example                   
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                                                4H                        
Experiment No.                                                            
                38     107     104        106     164                     
Substrate     AM-350 AM-350  AM-350                                       
                                  Ti/6A1/4V                               
                                        AM-350  AM-350                    
                                                     Ti/6A1/4V            
                                                           IN-718         
Temperature, °C.                                                   
                460    460     460        460     460                     
Pressure, Torr                                                            
                40      40     40         40      40                      
W Coating Conditions                                                      
H.sub.2, SCCM 3,000  3,000   3,000      3,000   3,000                     
Ar, SCCM      --     4,500   4,500      4,500   4,500                     
WF.sub.6, SCCM                                                            
                300    300     300        300     300                     
Time, Min.      10     15      15         15      25                      
Tungsten/Tungsten Carbide                                                 
Conditions                                                                
H.sub.2, SCCM 3,000  3,000   3,000      3,000   3,000                     
Ar, SCCM      --       300     300        300     300                     
WF.sub.6, SCCM                                                            
                300    300     300        300     300                     
DME, SCCM       40     30      50         40      40                      
Time, Min.      30     30      30         30      25                      
Coating Thickness, μm                                                  
Tungsten      8.0    8.5     9.0  8.0   9.0     16.9 17.1  16.9           
Tungsten/Tungsten Carbide                                                 
              16.0   12.5    11.5 11.0  11.0    14.5 13.3  11.7           
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
              0.50   0.68    0.78 0.73  0.81    1.17 1.29  1.44           
Thickness                                                                 
Surface Topography                                                        
              Smooth,                                                     
                     Smooth, A Few                                        
                             Smooth, A Few Fine                           
                                        Smooth, A                         
                                                Smooth, A Few Fine        
              Several Fine                                                
                     Fine Inter-                                          
                             Interconnected                               
                                        Few Fine                          
                                                Interconnected            
              Inter- Connected                                            
                             Cracks     Interconnected                    
                                                Cracks                    
              connected                                                   
                     Cracks             Cracks                            
              Cracks                                                      
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               Example 4I      Example 4J      Example 4K                 
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Experiment No.   177             167             160                      
Substrate      AM-350                                                     
                    Ti/6A1/4V                                             
                           IN-718                                         
                               AM-350                                     
                                    Ti/6A1/4V                             
                                           IN-718                         
                                               AM-350                     
                                                    Ti/6A1/4V             
                                                           IN-718         
Temperature, °C.                                                   
                 460             460             460                      
Pressure, Torr   40              40              40                       
W Coating Conditions                                                      
H.sub.2, SCCM  2,500           3,000           2,500                      
Ar, SCCM       6,000           4,500           6,000                      
WF.sub.6, SCCM   250             300             250                      
Time, Min.       25              25              25                       
Tungsten/Tungsten Carbide                                                 
Conditions                                                                
H.sub.2, SCCM  3,000           3,000           3,000                      
Ar, SCCM       2,000             300           2,000                      
WF.sub.6, SCCM   300             300             300                      
DME, SCCM        40              40              40                       
Time, Min.       35               20             15                       
Coating Thickness, μm                                                  
Tungsten       11.0 10.5   10.3                                           
                               15.5 15.7   15.0                           
                                               13.2 12.5   12.5           
Tungsten/Tungsten Carbide                                                 
               7.7  7.8    7.7 8.7  9.0    9.0 4.4  3.4    4.0            
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
               1.42 1.35   1.34                                           
                               1.78 1.74   1.67                           
                                               3.00 3.67   3.12           
Thickness                                                                 
Surface Topography                                                        
               Smooth, A Few Fine                                         
                               Smooth, A Few Fine                         
                                               Smooth, No Cracks          
               Interconnected  Interconnected                             
               Cracks          Cracks                                     
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     EXAMPLE FOR TUNGSTEN/TUNGSTEN CARBIDE (W+W 3  C) COATING 
     EXAMPLE 3 
     In this example, several specimens of AM-350, Ti/6Al/4V and IN-718 were coated simultaneously in a single run. The specimens were heated to a temperature of about 460° C. in the presence of flowing argon and at the reaction temperature a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min of hydrogen and 40 cc/min of DME was passed into the furnace over the specimens. The total pressure was maintained at 40 Torr, as shown in Table 2. The deposition was conducted for 40 minutes. 
     All the specimens were coated with a bright, smooth, adherent, coherent and uniform coating. The coating thickness on stainless steel specimens was ˜22 μm. The coating consisted of a mixture of W and W 3  C phases, as determined by x-ray diffraction. It was free of columnar grains. The coating had a smooth surface finish. However, the surface of the coating was heavily cracked, as shown in the FIG. 5. The coating had a hardness of 1788 Vickers, as shown in Table 3. The coating showed poor erosion resistance; the breakthrough time and erosion rate were 36 seconds and 42 seconds/mil, respectively. The weight loss during erosion test was 0.00036 g in 30 seconds. Extensive chipping of the coating was observed during the erosion test. Poor erosion resistance of the coating was probably due to presence of a network of cracks in the coating. 
     EXAMPLES FOR TUNGSTEN FOLLOWED BY TUNGSTEN/TUNGSTEN CARBIDE (W+W 3  C) COATING 
     EXAMPLE 4A 
     In this example, a two step coating process was used. Several AM-350, Ti/6Al/4V and IN-718 specimens were heated to a temperature of about 460° C. in the presence of flowing argon and at the reaction temperature a gaseous mixture of 300 cc/min WF 6 , and 3,000 cc/min of hydrogen was passed into the furnace over the specimens for 5 minutes to coat them with tungsten. After coating specimens with tungsten for 5 minutes, a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min hydrogen and 40 cc/min of DME was passed into the furnace for 55 minutes to provide tungsten/tungsten carbide coating. A total pressure was maintained at 40 Torr during tungsten as well as tungsten/tungsten carbide coating steps (see Table 2). 
     The stainless steel and Ti/6Al/4V specimens were coated with 2-3 μm thick tungsten followed by 27-28 μm thick tungsten/tungsten carbide coating as shown in Table 2. The tungsten/tungsten carbide top coat consisted of a mixture of W and W 3  C phases as shown in Table 3. The hardness values of the coating on AM-350 and Ti/6Al/4V are summarized in Table 3. The coating on AM-350 and Ti/6Al/4V showed the presence of a network of cracks. Erosion resistance of the coating was extremely poor, as shown in Table 3. Additionally, extensive chipping of the coating was observed during the erosion test. Poor erosion resistance of the coating was probably due to extensive cracking of the coating. 
     This example described that providing a very thin interlayer of tungsten did not help in improving erosion resistance of the overall composite coating. 
     EXAMPLE 4B 
     The CVD run described in Example 4A was repeated to provide tungsten followed by tungsten/tungsten carbide coatings. The reaction conditions used in tungsten and tungsten/tungsten carbide coating steps are summarized in Table 2. 
     The stainless steel specimens were coated with 3 μm tungsten followed by 25 μm of tungsten/tungsten carbide. The top coat consisted of a mixture of W and W 3  C phases. The coating showed the presence of a network of cracks. Erosion resistance of the coating improved slightly, but it was still extremely poor as shown in Table 3. Additionally, extensive chipping of the coating was observed during the erosion test. Poor erosion resistance was due to the presence of a network of cracks in the coating. 
     This example described that increasing the ratio of the thickness of the tungsten to the tungsten/tungsten carbide layer helped in improving the erosion resistance of the composite coating. 
     
                                           TABLE 3                                 
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               Example 2                                                  
                       Example 3                                          
                               Example 4A  Example 4B                     
                                                   Example                
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                                                   4C                     
Experiment No. 50      36      37          33      42                     
Substrate      AM-350  AM-350  AM-350                                     
                                     Ti/6A1/4V                            
                                           AM-350  AM-350                 
                                                         Ti/6A1/4V        
Coating Composition                                                       
               W       W + W.sub.3 C                                      
                               W + W.sub.3 C                              
                                           W + W.sub.3 C                  
                                                   W + W.sub.3 C          
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer 455 ± 50                                                
                       --      --          --      --    --               
Tungsten/Tungsten Carbide                                                 
               --      1788 ± 130                                      
                               2276 ± 103                              
                                     2150 ± 128                        
                                           2333 ± 165                  
                                                   --    --               
Layer                                                                     
Erosion Resistance                                                        
Breakthrough Time, Secs                                                   
               3       36      42    34    68      43    45               
Calculated Erosion Rate,                                                  
               6       42      40    31    69      84    82               
secs/mil                                                                  
Weight Loss in 30 seconds, g                                              
               --      0.00036 --    --    0.00040 --    --               
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              Example 4D                                                  
                     Example 4E                                           
                            Example 4F  Example 4G                        
                                                Example                   
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                                                4H                        
Experiment No.                                                            
              38     107    104         106      164                      
Substrate     AM-350 AM-350 AM-350                                        
                                  Ti/6A1/4V                               
                                        AM-350  AM-350                    
                                                     Ti/6A1/4V            
                                                           IN-718         
Coating Composition                                                       
              W + W.sub.3 C                                               
                     W + W.sub.3 C                                        
                            W + W.sub.3 C                                 
                                        W + W.sub.3 C                     
                                                W + W.sub.3 C             
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer                                                            
              --     --     --    --    --      --   --    --             
Tungsten/Tungsten Carbide                                                 
              2164 ± 264                                               
                     2395 ± 15                                         
                            2361 ± 103                                 
                                  2328 ± 203                           
                                        2470 ± 53                      
                                                2424 2175  2539           
Layer                                                                     
Erosion Resistance                                                        
Breakthrough Time, Secs                                                   
              89      93    115    93    95      128  115   109           
Calculated Erosion Rate,                                                  
              141    188    255   215   232      224  220   237           
secs/mil                                                                  
Weight Loss in 30 seconds, g                                              
              0.00014                                                     
                     --     --    --    --      0.00012                   
                                                     --    --             
__________________________________________________________________________
                 Example 4I     Example 4J      Example                   
__________________________________________________________________________
                                                4K                        
Experiment No.    177            167            169                       
Substrate        AM-350                                                   
                      Ti/6A1/4V                                           
                            IN-718                                        
                                AM-350                                    
                                     Ti/6A1/4V                            
                                            IN-718                        
                                                AM-350                    
                                                     Ti/6A1/4V            
                                                           IN-718         
Coating Composition                                                       
                 W + W.sub.3 C  W + W.sub.3 C   W + W.sub.3 C             
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer   --   --    --  --   --     --  --   --    --             
Tungsten/Tungsten Carbide Layer                                           
                 1930 1997  1971                                          
                                2508 2516   2567                          
                                                --   --    --             
Erosion Resistance                                                        
Breakthrough Time, Secs                                                   
                  69   70    70  94   93     96  45   34    42            
Calculated Erosion Rate, secs/mil                                         
                  227  228   231                                          
                                 274  262    271                          
                                                260  252   266            
Weight Loss in 30 seconds, g                                              
                 --   --    --  --   --     --  0.00014                   
                                                     --    --             
__________________________________________________________________________
 
    
     EXAMPLE 4C 
     The CVD run described in Example 4A was repeated to provide tungsten followed by slightly thinner tungsten/tungsten carbide coatings. The reaction conditions used in tungsten and tungsten/tungsten carbide coating steps are summarized in Table 2. 
     The thicknesses of the tungsten and the tungsten/tungsten carbide layers obtained on AM-350 and Ti/6Al/4V are summarized in Table 2. The top coat of the coating consisted of a mixture of W and W 3  C phases. The coating, once again, showed the presence of a network of cracks. However, the crack density was considerably lower than that observed in Examples 3, 4A and 4B. This suggested that the tungsten interlayer was helpful in reducing the crack density. The erosion resistance of the coating was considerably better then that of the coatings obtained in Examples 3, 4A and 4B (see Table 3). The extent of chipping observed during the erosion test also reduced considerably. 
     This example showed that increasing the ratio of the thickness of the tungsten layer to that of the tungsten/tungsten carbide layer from ˜0.07 to 0.12 in Examples 4A and 4B to ˜0.2 to 0.3 in this example unexpectedly reduced cracks in the coating and improved its erosion resistance. 
     EXAMPLE 4D 
     The CVD run described in Example 4A was once again repeated to provide tungsten followed by tungsten/tungsten carbide coating. The reaction conditions used for depositing tungsten and tungsten/tungsten carbide coatings were selected in such a way to provide a ratio of the thickness of the tungsten to the tungsten/tungsten carbide layers of ˜0.5 (see Table 2). 
     The coating showed the presence of cracks, but the crack density was greatly reduced. The coating, surprisingly, showed superior erosion resistance compared to Examples 3 and 4A to 4C. Furthermore, the composite coating obtained in this example showed significantly lower weight loss in the erosion resistance test than Example 3. The chipping of the coating observed during the erosion test was reduced dramatically as well. This example, therefore, clearly demonstrated the importance of the tungsten interlayer in reducing cracks and improving erosion resistance of the composite coating. 
     EXAMPLE 4E 
     In this example, reaction conditions for coating tungsten followed by tungsten/tungsten carbide were selected in such a way to provide a ratio of the thickness of the tungsten to the tungsten/tungsten carbide layers of ˜0.68 (see Table 2). 
     The composite coating showed the presence of only a few fine cracks (see FIG. 6). Etched cross section of the coating clearly showed columnar tungsten interlayer followed by non-columnar W+W 3  C coating. The composite coating demonstrated superior erosion performance as shown in Table 3. 
     This example clearly demonstrated the importance of the tungsten interlayer in reducing cracks and improving erosion resistance of the composite coating. 
     EXAMPLE 4F 
     In this example, reaction conditions for coating tungsten followed by tungsten/tungsten carbide were selected in such a way to slightly increase the thickness ratio (see Table 2). 
     Once again, the coating on AM-350 and Ti/6Al/4V showed presence of a few cracks. Erosion resistance of the composite coating summarized in Table 3 was better than Example 4E. This example also demonstrated the importance of the tungsten interlayer in reducing cracks and improving erosion resistance. 
     EXAMPLE 4G 
     The CVD run described in Example 4F was repeated using reaction conditions summarized in Table 2 to provide slightly higher tungsten to tungsten/tungsten carbide coating thickness ratio. 
     Once again, the coating showed presence of a few fine cracks. Etched cross section of the coating clearly showed the presence of columnar tungsten interlayer and non-columnar tungsten/tungsten carbide top coat. The erosion resistance of the coating was very similar to that observed in Example 4F. 
     EXAMPLE 4H 
     The CVD run described in Example 4G was repeated using reaction conditions summarized in Table 2 to provide even higher tungsten to tungsten/tungsten carbide coating thickness ratio. 
     The coating on AM-350, Ti/6Al/4V and IN-718 showed presence of a few cracks. The erosion resistance of the coating was very similar to that noted in Examples 4F and 4G. The weight loss during the erosion test was considerably lower than that observed earlier. 
     This example, therefore, showed the importance of the tungsten interlayer in reducing the cracks in the coating and in improving the erosion resistance of the coating. It also showed that increasing the thickness of the tungsten interlayer to ˜16 μm without increasing that of the tungsten/tungsten carbide did not help in further increasing the erosion resistance of the coating. 
     EXAMPLE 4I 
     The CVD run described in Example 4H was repeated to provide even higher tungsten to tungsten/tungsten carbide coating thickness ratio. The higher ratio was obtained by reducing the thickness of the tungsten/tungsten carbide layer. 
     The coating showed the presence of a few very fine cracks. The erosion resistance of the composite coating was similar to that noted in Example 4H (see Table 3). This example, once again, showed the importance of the tungsten interlayer in reducing cracks and improving erosion resistance of the composite coating. It also showed that the breakthrough time was dependent on the thickness of the tungsten/tungsten carbide layer; whereas, the erosion rate was independent of it. 
     EXAMPLE 4J 
     The CVD run described in Example 4I was repeated to provide even higher tungsten to tungsten/tungsten carbide coating thickness ratio. The higher ratio was obtained by increasing the thickness of the tungsten interlayer. 
     The coating, once again, showed presence of a few very fine cracks. The erosion resistance of the composite coating was slightly higher than that noted earlier (see Table 3). This example also showed the importance of the tungsten interlayer. 
     EXAMPLE 4K 
     The CVD run described in Example 4J was repeated to provide even higher tungsten to tungsten/tungsten carbide coating thickness ratio. 
     The coating, unexpectedly, was found to be absolutely crack free, as shown in the FIG. 7. The erosion resistance of the coating was comparable to the data noted earlier. Weight loss during erosion test was also comparable to the data noted earlier. 
     This example demonstrated that a crack-free coating could be obtained by manipulating the ratio of the thickness of the tungsten and the tungsten/tungsten carbide layers. This is an unexpected and significant finding. 
     Discussion on Tungsten Followed by W+W 3  C Coating 
     Because of extensive cracking, the W+W 3  C coating without a W interlayer provided very little erosion protection to the base metal. In this case, erosion occurs preferentially at cracks which causes chipping and flaking of large pieces of the coating. The cracks are believed to occur during the deposition and/or cool-down due to the build up in stresses within the coating. Surprisingly, the cracks in the coating can be minimized or even eliminated by providing a tungsten interlayer. It is believed that the function of the tungsten interlayer is to accommodate stresses that build up in the coating during the deposition and/or cool-down. The ability to accommodate stress may be in part due to the columnar structure of the tungsten layer, since the compliance of this layer is probably very anisotropic. The amount of coating stress which can be accommodated by the tungsten interlayer will depend on its thickness. However, the physical dimensions over which a significant amount of stress accommodation will occur will be limited so that the stresses which are present in the outer layer of a thick tungsten/tungsten carbide coating will be virtually unaffected by the presence of the interlayer. Consequently, both the thickness of the tungsten/tungsten carbide layer and the ratio of the W to the W+W 3  C thickness are important in obtaining a crack-free composite coating. 
     The presence of a tungsten interlayer, in which the thickness of the tungsten interlayer is at least 3 μm and the ratio of W to W+W 3  C thickness is at least above 0.3, is necessary to increase the erosion resistance of the composite coating. However, the presence of a tungsten interlayer, in which the ratio of W to W+W 3  C thickness is at least 0.6, is necessary for the optimum erosion and abrasion wear performance. 
     The tungsten interlayer also effects the performance of the tungsten/tungsten carbide coating in a scratch test. In this test, a diamond stylus is dragged across the surface of the sample at a constantly increasing load. The load at which coating loss begins to occur and the extent of coating loss can generally be correlated to the performance of a coating in the erosive and abrasive wear applications. The tungsten/tungsten carbide coating with no tungsten interlayer as described in Example 3 showed extensive coating loss in the 30-40 Newton load range in the scratch test, as shown in the FIG. 8. When a tungsten interlayer is present as described in Example 4E, the loss of the coating in the same load range is significantly reduced, as demonstrated in the FIG. 9; consequently, the presence of a tungsten interlayer is important for erosive and abrasive wear applications. 
     Due to the preferential erosive and abrasive wear, which occurs at cracks, a crack-free coating may be highly desirable under certain wear conditions. This conclusion would be especially true in situations where the substrate needs to be completely protected, such as in corrosive-wear environments. A crack-free coating will also be important when a smooth wear surface is required. This crack-free coating was unexpectedly obtained by significantly increasing the ratio of the thickness of the W to the W+W 3  C layer. Although Example 4K was produced by depositing a thin layer of W+W 3  C, in theory, a crack-free coating could also be produced by depositing a very thick tungsten interlayer. 
     EXAMPLES FOR TUNGSTEN/TUNGSTEN CARBIDE (W+W 2  C+W 3  C) COATING 
     EXAMPLE 5A 
     In this example, several AM-350 specimens were coated in a run. The specimens were heated to a temperature of about 460° C. in the presence of flowing argon gas and at the reaction temperature a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min of hydrogen and 55 cc/min of DME was passed into the furnace over the specimens. The total pressure was maintained at 40 Torr, as shown in Table 4. The deposition was conducted for 20 minutes. 
     All the specimens were coated with a bright, smooth, adherent, coherent and uniform coating. The coating thickness was ˜7 μm (see Table 4). It consisted of a mixture of W, W 2  C and W 3  C phases, which was considerably different than observed in Example 3. It was free of columnar grains. The micro-structure of the coating consisted of a layered structure. The coating had a smooth surface finish. However, the surface of the coating was cracked, as shown in the FIG. 10. The crack density was surprisingly lower than that observed in Example 3. The coating had a hardness of 2248 Vickers (Table 5). The coating showed poor erosion resistance; the breakthrough time and erosion rate were 21 seconds and 76 seconds/mil. The erosion resistance, however, was much higher than Example 3. The weight loss during erosion test was 0.00042 g in 30 seconds, which was very similar to that noted in Example 3. Extensive chipping of the coating was observed during the erosion test. Poor erosion resistance of the coating was probably due to the presence of a network of cracks in the coating. 
     EXAMPLE 5B 
     The CVD run described in Example 5A was repeated with Ti/6Al/4V specimens using reaction conditions described in Table 4. The specimens were coated with a bright, smooth, adherent, coherent and uniform ˜9.5 μm thick coating. It consisted of a mixture of W, W 2  C and W 3  C phases. It was free of columnar grains. It had a smooth surface finish. It consisted of a network of cracks on the surface. It showed poor erosion resistance (see Table 5), but the erosion resistance was 
     
                                           TABLE 4                                 
__________________________________________________________________________
              Example 5A                                                  
                      Example 5B                                          
                              Example 6A Example 6B Example               
__________________________________________________________________________
                                                    6C                    
Experiment No.                                                            
                51      140     70         72         77                  
Substrate     AM-350  Ti/6A1/4V                                           
                              AM-350                                      
                                   Ti/6A1/4V                              
                                         AM-350                           
                                              Ti/6A1/4V                   
                                                    AM-350                
                                                         Ti/6A1/4V        
Temperature, °C.                                                   
                460     460     460        465        465                 
Pressure, Torr                                                            
                40      40      40         40         40                  
W Coating Conditions                                                      
H.sub.2, SCCM --      --      3,000      3,000      3,000                 
Ar, SCCM      --      --      4,500      4,500      4,500                 
WF.sub.6, SCCM                                                            
              --      --        300        300        300                 
Time, Min.    --      --         3       4.5          10                  
Tungsten Carbide Conditions                                               
H.sub.2, SCCM 3,000   3,000   3,000      3,000      3,250                 
Ar, SCCM      --      1,800   --         --         --                    
WF.sub.6, SCCM                                                            
                300     300     300        300        300                 
DME, SCCM       55      55      55         55         60                  
Time, Min.      20      35      20         20         45                  
Coating Thickness, μm                                                  
Tungsten      --      --      3.0  3.4   5.0  5.0   8.0  8.0              
Tungsten/Tungsten Carbide                                                 
              7.0     9.5     8.4  9.0   13.0 9.5   20.0 15.0             
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
              --      --      0.36 0.38  0.38 0.52  0.40 0.53             
Thickness                                                                 
Surface Topography                                                        
              Smooth, Many                                                
                      Smooth, Many                                        
                              Smooth, A Few Inter-                        
                                         Smooth, A Few Fine               
                                                    Smooth, A             
              Interconnected                                              
                      Interconnected                                      
                              connected Cracks                            
                                         Cracks     Few Cracks            
              Cracks  Cracks                                              
__________________________________________________________________________
               Example 6D  Example 6E  Example 6F  Example                
__________________________________________________________________________
                                                   6G                     
Experiment No.   53          103         82          111                  
Substrate      AM-350                                                     
                    Ti/6A1/4V                                             
                           AM-350                                         
                                Ti/6A1/4V                                 
                                       AM-350                             
                                            Ti/6A1/4V                     
                                                   AM-350                 
                                                        Ti/6A1/4V         
Temperature, °C.                                                   
                 460         460         460         465                  
Pressure, Torr   40          40          40          40                   
W Coating Conditions                                                      
H.sub.2, SCCM  3,000       3,000       3,000       3,000                  
Ar, SCCM       4,000       4,500       4,500       4,500                  
WF.sub.6, SCCM   300         300         300         300                  
Time, Min.        7          15          10          15                   
Tungsten Carbide Conditions                                               
H.sub.2, SCCM  3,000       3,000       3,500       3,000                  
Ar, SCCM       --            300       --            300                  
WF.sub.6, SCCM   300         300         350         300                  
DME, SCCM        55          60          65          50                   
Time, Min.       20          40          30          30                   
Coating Thickness, μm                                                  
Tungsten       6.0  6.0    9.2  7.7    9.0  8.6    9.6  8.8               
Tungsten/Tungsten Carbide                                                 
               9.5  9.5    12.3 12.3   16.0 11.5   11.5 12.0              
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
               0.63 0.63   0.74 0.63   0.56 0.75   0.83 0.73              
Thickness                                                                 
Surface Topography                                                        
               Smooth,                                                    
                    Smooth,                                               
                           Smooth, A Few Fine                             
                                       Smooth, A Few Fine                 
                                                   Smooth, A Few Fine     
               No   A Few  Cracks      Cracks      Cracks                 
               Cracks                                                     
                    Cracks                                                
__________________________________________________________________________
               Example 6H  Example 6I  Example 6J  Example                
__________________________________________________________________________
                                                   6K                     
Experiment No.   102         52          56          76                   
Substrate      AM-350                                                     
                    Ti/6A1/4V                                             
                           AM-350                                         
                                Ti/6A1/4V                                 
                                       AM-350                             
                                            Ti/6A1/4V                     
                                                   AM-350                 
                                                        Ti/6A1/4V         
Temperature, °C.                                                   
                 460         460         460         465                  
Pressure, Torr   40          40          40          40                   
W Coating Conditions                                                      
H.sub.2, SCCM  3,000       3,000       3,000       3,000                  
Ar, SCCM       4,500       4,000       4,500       4,000                  
WF.sub.6, SCCM   300         300         300         300                  
Time, Min.       15          10          10          30                   
Tungsten Carbide Conditions                                               
H.sub.2, SCCM  3,000       3,000       3,000       3,000                  
Ar, SCCM         300       --          --          --                     
WF.sub.6, SCCM   300         300         300         300                  
DME, SCCM        60          55          55          55                   
Time, Min.       30          20          15          45                   
Coating Thickness, μm                                                  
Tungsten       10.0 8.1     8.0 7.5    7.1  7.5    19.0 16.0              
Tungsten/Tungsten Carbide                                                 
               8.0  8.0    10.0 7.5    6.2  6.0    11.0 9.0               
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
               1.25 1.01   0.80 1.00   1.14 1.25   1.75 1.78              
Thickness                                                                 
Surface Topography                                                        
               Smooth, No Cracks                                          
                           Smooth, No Cracks                              
                                       Smooth, No Cracks                  
                                                   Smooth, No             
__________________________________________________________________________
                                                   Cracks                 
 
    
     
                                           TABLE 5                                 
__________________________________________________________________________
                  Example 5A                                              
                            Example 5B Example 6A  Example                
__________________________________________________________________________
                                                   6B                     
Experiment No.    51         140       70          72                     
Substrate         AM-350    Ti/6A1/4V  AM-350                             
                                             Ti/6A1/4V                    
                                                   AM-350                 
                                                         Ti/6A1/4V        
Coating Composition                                                       
                  W + W.sub.2 C + W.sub.3 C                               
                            W + W.sub.2 C + W.sub.3 C                     
                                       W + W.sub.2 C + W.sub.3 C          
                                                   W + W.sub.2 C +        
                                                   W.sub.3 C              
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer    --        --         --    --    --    --               
Tungsten/Tungsten Carbide Layer                                           
                  2248 ± 70                                            
                            2191       2078 ± 49                       
                                             --    2253                   
                                                         ---. 73          
Erosion Resistance                                                        
Breakthrough Time, secs                                                   
                  21         44        104   131   160   147              
Calculated Erosion Rate,                                                  
                  76         117       314   370   313   394              
secs/mil                                                                  
Weight Loss in    0.00042   0.0028     0.00012                            
                                             0.00015                      
                                                   --    --               
30 seconds, g                                                             
__________________________________________________________________________
                           Example 6C  Example 6D  Example                
__________________________________________________________________________
                                                   6E                     
          Experiment No.   77          53          103                    
           Substrate       AM-350                                         
                                 Ti/6A1/4V                                
                                       AM-350                             
                                             Ti/6A1/4V                    
                                                   AM-350                 
                                                         Ti/6A1/4V        
          Coating Composition                                             
                           W + W.sub.2 C + W.sub.3 C                      
                                       W + W.sub.2 C + W.sub.3 C          
                                                   W + W.sub.2 C +        
                                                   W.sub.3 C              
          Coating Hardness, Kg/mm.sup.2                                   
          Tungsten Layer   --    --    --    --    --    --               
          Tungsten/Tungsten Carbide Layer                                 
                           2102 ± 101                                  
                                 --    2359 ± 114                      
                                             2215 ± 194                
                                                   2224                   
                                                         2469 ± 53     
          Erosion Resistance                                              
          Breakthrough Time, secs                                         
                           218   201   119   --    158   137              
          Calculated Erosion Rate,                                        
                           278   340   318   --    325   283              
          secs/mil                                                        
          Weight Loss in   --    --    --    --    --    --               
          30 seconds, g                                                   
__________________________________________________________________________
                           Example 6F  Example 6G  Example                
__________________________________________________________________________
                                                   6H                     
          Experiment No.   82          111         102                    
          Substrate        AM-350                                         
                                 Ti/6A1/4V                                
                                       AM-350                             
                                             Ti/6A1/4V                    
                                                   AM-350                 
                                                         Ti/6A1/4V        
          Coating Composition                                             
                           W + W.sub.2 C + W.sub.3 C                      
                                        W + W.sub.2 C + W.sub.3 C         
                                                   W + W.sub.2 C +        
                                                   W.sub.3 C              
          Coating Hardness, Kg/mm.sup.2                                   
          Tungsten Layer   --    --    --    --    --    --               
          Tungsten/Tungsten Carbide Layer                                 
                           2220 ± 100                                  
                                 --    3097 ± 210                      
                                             3156 ± 76                 
                                                   2167                   
                                                         2324 ± 50     
          Erosion Resistance                                              
          Breakthrough Time, secs                                         
                           209   128   146   123    67    66              
          Calculated Erosion Rate,                                        
                           332   284   322   260   212   208              
          secs/mil                                                        
          Weight Loss in   --    --    --    --    --    --               
          30 seconds, g                                                   
__________________________________________________________________________
                           Example 6I  Example 6J  Example                
__________________________________________________________________________
                                                   6K                     
          Experiment No.   52          56          76                     
          Substrate        AM-350                                         
                                 Ti/6A1/4V                                
                                       AM-350                             
                                             Ti/6A1/4V                    
                                                   AM-350                 
                                                         Ti/6A1/4V        
          Coating Composition                                             
                           W + W.sub.2 C + W.sub.3 C                      
                                       W + W.sub.2 C + W.sub.3 C          
                                                   W + W.sub.2 C +        
                                                   W.sub.3 C              
          Coating Hardness, Kg/mm.sup.2                                   
           Tungsten Layer  --    --    --    --    --    --               
          Tungsten/Tungsten Carbide Layer                                 
                           2091 ± 101                                  
                                 2155 ± 20                             
                                       2078 ± 66                       
                                             --    --    2091 ± 98     
          Erosion Resistance                                              
          Breakthrough Time, secs                                         
                           125    73    58    91   158   150              
          Calculated Erosion Rate,                                        
                           317   248   237   383   364   423              
          secs/mil                                                        
          Weight Loss in   0.00012                                        
                                 0.00014                                  
                                       --    --    --    --               
          30 seconds, g                                                   
__________________________________________________________________________
 
    
     
                                           TABLE 6                                 
__________________________________________________________________________
              Example 7A   Example 7B        Example 7C                   
__________________________________________________________________________
Experiment No.                                                            
                69           121               115                        
Substrate     AM-350                                                      
                    Ti/6A1/4V                                             
                           AM-350                                         
                                 Ti/6A1/4V                                
                                        IN-718                            
                                             AM-350                       
                                                   Ti/6A1/4V              
                                                          IN-718          
Temperature, °C.                                                   
                460          440               460                        
Pressure, Torr                                                            
                40           40                40                         
W Coating Conditions                                                      
H.sub.2, SCCM 3,000        2,500             3,000                        
Ar, SCCM      4,500        5,500             4,500                        
WF.sub.6, SCCM                                                            
                300          250               300                        
Time, Min.      15           15                10                         
Tungsten/Tungsten                                                         
Carbide Conditions                                                        
H.sub.2, SCCM 3,000        3,000             3,000                        
Ar, SCCM      --           1,800               300                        
WF.sub.6, SCCM                                                            
                300          300               300                        
DME, SCCM       85           90                90                         
Time, Min.      70           115               40                         
Coating Thickness, μm                                                  
Tungsten      8.0   10.0   6.8   6.0    6.3  4.8   4.6    5.0             
Tungsten/Tungsten Carbide                                                 
              23.0  23.0   17.0  16.0   15.0 11.0  10.0   10.0            
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
              0.35  0.43   0.40  0.37   0.42 0.44  0.46   0.50            
Thickness                                                                 
Surface Topography                                                        
              Smooth, A few fine                                          
                           Smooth, a few Fine                             
                                             Smooth,                      
                                                   Smooth,                
                                                          Smooth,         
              Interconnected                                              
                           Cracks            No Cracks                    
                                                   Fine                   
                                                          Noacks          
              Cracks                                      Cracks          
__________________________________________________________________________
               Example 7D Example 7E     Example 7F Example               
__________________________________________________________________________
                                                    7G                    
Experiment No.   95         120            119        91                  
Substrate      AM-350                                                     
                    Ti/6A1/4V                                             
                          AM-350                                          
                               Ti/6A1/4V                                  
                                     IN-718                               
                                         AM-350                           
                                              Ti/6A1/4V                   
                                                    AM-350                
                                                         Ti/6A1/4V        
Temperature, °C.                                                   
                 460        445            450        460                 
Pressure, Torr   40         40             40         40                  
W Coating Conditions                                                      
H.sub.2, SCCM  3,000      2,500          2,500      3,000                 
Ar, SCCM       4,500      5,500          5,500      4,500                 
WF.sub.6, SCCM   300        250            250        300                 
Time, Min.       10         15             15         10                  
Tungsten/Tungsten                                                         
Carbide Conditions                                                        
H.sub.2, SCCM  3,000      3,000          3,000      3,000                 
Ar, SCCM         300      1,800          1,500        300                 
WF.sub.6, SCCM   300        300            300        300                 
DME, SCCM        90         90             90         90                  
Time, Min.       50         95             80         40                  
Coating Thickness, μm                                                  
Tungsten       6.6  4.4   6.8  6.2   6.4 6.8  6.8   5.6  5.0              
Tungsten/Tungsten Carbide                                                 
               14.8 12.0  13.0 12.7  10.6                                 
                                         12.4 12.8  10.0 10.0             
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
               0.44 0.37  0.52 0.49  0.60                                 
                                         0.55 0.53  0.56 0.50             
Thickness                                                                 
Surface Topography                                                        
               Smooth, Fine Cracks                                        
                          Smooth, No Cracks                               
                                         Smooth, No Cracks                
                                                    Smooth, No            
__________________________________________________________________________
                                                    Cracks                
               Example 7H    Example 7I          Example                  
__________________________________________________________________________
                                                 7J                       
Experiment No.   97            122                 68                     
Substrate      AM-350 Ti/6A1/4V                                           
                             AM-350 Ti/6A1/4V                             
                                           IN-718                         
                                                 AM-350                   
                                                       Ti/6A1/4V          
Temperature, °C.                                                   
                 460           460                 460                    
Pressure, Torr   40            40                  40                     
W Coating Conditions                                                      
H.sub.2, SCCM  3,000         2,500               3,000                    
Ar, SCCM       4,500         5,500               4,500                    
WF.sub.6 , SCCM                                                           
                 300           250                 300                    
Time, Min.       15            25                  30                     
Tungsten/Tungsten                                                         
Carbide Conditions                                                        
H.sub.2, SCCM  3,000         3,000               3,000                    
Ar, SCCM         300         1,500               --                       
WF.sub.6, SCCM   300           300                 300                    
DME, SCCM        90            90                  85                     
Time, Min.       50            85                  70                     
Coating Thickness, μm                                                  
Tungsten       9.0    7.0    12.4   12.3   12.1  25.0  23.0               
Tungsten/Tungsten Carbide                                                 
               15.0   11.5   19.1   18.3   17.5  27.0  26.0               
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
               0.60   0.61   0.65   0.67   0.69  0.93  0.88               
Thickness                                                                 
Surface Topography                                                        
               Smooth,                                                    
                      Smooth,                                             
                             Smooth, Cracks      Smooth, Cracks           
               Cracks No Cracks                                           
__________________________________________________________________________
               Example 7K     Example 7L Example 7M Example               
__________________________________________________________________________
                                                    7N                    
Experiment No.   123            100        101        63                  
Substrate      AM-350                                                     
                    Ti/6A1/4V                                             
                          IN-718                                          
                              AM-350                                      
                                   Ti/6A1/4V                              
                                         AM-350                           
                                              Ti/6A1/4V                   
                                                    AM-350                
                                                         Ti/6A1/4V        
Temperature, °C.                                                   
                 460            460         460       460                 
Pressure, Torr   40             40         40         40                  
W Coating Conditions                                                      
H.sub.2, SCCM  2,500          3,000      3,000      3,000                 
Ar, SCCM       5,500          4,500      4,500      4,500                 
WF.sub.6, SCCM   250            300        300        300                 
Time, Min.       18             15         15         10                  
Tungsten/Tungsten                                                         
Carbide Conditions                                                        
H.sub.2, SCCM  3,000          3,000      3,000      3,000                 
Ar, SCCM       1,800            300        300        300                 
WF.sub.6, SCCM   300            300        300        300                 
DME, SCCM        90             80         70         85                  
Time, Min.       65             40         40         20                  
Coating Thickness, μm                                                  
Tungsten       8.8  9.0   9.2 12.0 10.4  11.2 9.2   7.5  6.7              
Tungsten/Tungsten Carbide                                                 
               12.7 13.1  12.3                                            
                              11.6 10.4  10.0 9.2   7.7  6.5              
Tungsten Coating Thickness                                                
Tungsten/Tungsten Carbide                                                 
               0.69 0.68  0.75                                            
                              1.03 1.00  1.12 1.00  0.97 1.03             
Thickness                                                                 
Surface Topography                                                        
               Smooth, No Cracks                                          
                              Smooth, No Cracks                           
                                          Smooth, No Cracks               
                                                    Smooth, No            
__________________________________________________________________________
                                                    Cracks                
                                         Example 7O Example               
__________________________________________________________________________
                                                    7P                    
                           Experiment No.                                 
                                           65         90                  
                           Substrate     AM-350                           
                                              Ti/6A1/4V                   
                                                    AM-350                
                                                         Ti/6A1/4V        
                           Temperature, °C.                        
                                           460        460                 
                           Pressure, Torr                                 
                                           40         40                  
                           W Coating Conditions                           
                           H.sub.2, SCCM 3,000      3,000                 
                           Ar, SCCM      4,500      4,500                 
                           WF.sub.6, SCCM                                 
                                           300        300                 
                           Time, Min.       5         10                  
                           Tungsten/Tungsten                              
                           Carbide Conditions                             
                           H.sub.2, SCCM 3,000      3,000                 
                           Ar, SCCM        300        300                 
                           WF.sub.6, SCCM                                 
                                           300        300                 
                           DME, SCCM       90         90                  
                           Time, Min.      20         30                  
                           Coating Thickness, μm                       
                           Tungsten      4.0  4.0   5.6  5.2              
                           Tungsten/Tungsten Carbide                      
                                         6.2  5.9   7.6  7.4              
                           Tungsten Coating Thickness                     
                           Tungsten/Tungsten Carbide                      
                                         0.64 0.68  0.74 0.70             
                           Thickness                                      
                           Surface Topography                             
                                         Smooth, No Cracks                
                                                    Smooth, No            
__________________________________________________________________________
                                                    Cracks                
 
    
     
                                           TABLE 7                                 
__________________________________________________________________________
               Example 7A  Example 7B         Example 7C                  
__________________________________________________________________________
Experiment No. 69          121                115                         
Substrate      AM-350                                                     
                     Ti/6A1/4V                                            
                           AM-350                                         
                                 Ti/6A1/4V                                
                                        IN-718                            
                                              AM-350                      
                                                    Ti/6A1/4V             
                                                           IN-718         
Coating Composition                                                       
               W + W.sub.2 C                                              
                           W + W.sub.2 C      W + W.sub.2 C               
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer --    --    --    --     --    --    --     --             
Tungsten/Tungsten                                                         
               3060 ± 230                                              
                     2792 ± 60                                         
                           2470 ± 53                                   
                                 2432 ±53                              
                                        2588 ± 57                      
                                              2850 ± 66                
                                                    2947                  
                                                           ---. 71        
Carbide Layer                                                             
Erosion Resistance                                                        
Breakthrough Time, secs                                                   
               --    304   201   214    183   135   126    116            
Calculated Erosion Rate,                                                  
               --    336   300   339    310   312   319    295            
secs/mil                                                                  
Weight Loss in --    --    0.00018                                        
                                 0.00020                                  
                                        0.00024                           
                                              --    --     --             
30 seconds, g                                                             
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              Example 7D    Example 7E           Example                  
__________________________________________________________________________
                                                 7F                       
Experiment No.                                                            
              95            120                  119                      
Substrate     AM-350 Ti/6A1/4V                                            
                            AM-350 Ti/6A1/4V                              
                                          IN-718 AM-350 Ti/6A1/4V         
Coating Composition                                                       
              W + W.sub.2 C W + W.sub.2 C        W + W.sub.2 C            
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer                                                            
              --     --     --     --     --     --     --                
Tungsten/Tungsten                                                         
              2991 ± 63                                                
                     3106 ± 148                                        
                            2628 ± 15                                  
                                   2507 ± 20                           
                                          2402 ± 160                   
                                                 2469 ± 53             
                                                        2395 ± 15      
Carbide Layer                                                             
Erosion Resistance                                                        
Breakthrough Time, secs                                                   
              203    149    132    135    112    165    154               
Calculated Erosion Rate,                                                  
              349    311    258    270    284    325    306               
secs/mil                                                                  
Weight Loss in                                                            
              --     --     --     --     --     --     --                
30 seconds, g                                                             
__________________________________________________________________________
              Example 7G    Example 7H    Example 7I                      
__________________________________________________________________________
Experiment No.                                                            
              91            97            122                             
Substrate     AM-350 Ti/6A1/4V                                            
                            AM-350 Ti/6A1/4V                              
                                          AM-350 Ti/6A1/4V                
                                                        IN-718            
Coating Composition                                                       
              W + W.sub.2 C W +  W.sub.2 C                                
                                          W + W.sub.2 C                   
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer                                                            
              --     --     --     --     --     --     --                
Tungsten/Tungsten                                                         
              3089 ± 89                                                
                     2767 ± 188                                        
                            2908 ± 213                                 
                                   2758 ± 15                           
                                          2472 ± 110                   
                                                 2507 ± 15             
                                                        2432 ± 53      
Carbide Layer                                                             
Erosion Resistance                                                        
Breakthrough Time, secs                                                   
              124    113    182    150    259    234    248               
Calculated Erosion Rate,                                                  
              315    288    308    331    345    325    361               
secs/mil                                                                  
Weight Loss in                                                            
              --     --     --     --     --     --     --                
30 seconds, g                                                             
__________________________________________________________________________
              Example 7J    Example 7K           Example                  
__________________________________________________________________________
                                                 7L                       
Experiment No.                                                            
              68            123                  100                      
Substrate     AM-350 Ti/6A1/4V                                            
                            AM-350 Ti/6A1/4V                              
                                          IN-718 AM-350 Ti/6A1/4V         
Coating Composition                                                       
              W + W.sub.2 C W + W.sub.2 C        W + W.sub.2 C            
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer                                   --     --                
Tungsten/Tungsten                                                         
              2758 ± 31                                                
                     2689 ± 159                                        
                            2398 ± 109                                 
                                   2324 ± 50                           
                                          2511 ± 116                   
                                                 2758 ± 15             
                                                        2758 ± 15      
Carbide Layer                                                             
Erosion Resistance                                                        
Breakthrough Time, secs                                                   
              356    325    162    172    143    144    120               
Calculated Erosion Rate,                                                  
              335    318    325    328    295    316    293               
secs/mil                                                                  
Weight Loss in                                                            
              --     --     0.00016                                       
                                   0.00018                                
                                          0.00014                         
                                                 --     --                
30 seconds, g                                                             
__________________________________________________________________________
               Example 7M  Example 7N  Example 7O  Example                
__________________________________________________________________________
                                                   7P                     
Experiment No. 101         62          65          90                     
Substrate      AM-350                                                     
                     Ti/6A1/4V                                            
                           AM-350                                         
                                 Ti/6A1/4V                                
                                       AM-350                             
                                             Ti/6A1/4V                    
                                                   AM-350                 
                                                         Ti/6A1/4V        
Coating Composition                                                       
               W + W.sub.2 C                                              
                           W + W.sub.2 C                                  
                                       W + W.sub.2 C                      
                                                   W + W.sub.2 C          
Coating Hardness, Kg/mm.sup.2                                             
Tungsten Layer --    --    --    --    --    --    --    --               
Tungsten/Tungsten                                                         
               2434 ± 102                                              
                     2510 ± 95                                         
                           2746 ± 51                                   
                                 --    2758 ± 50                       
                                             --    2660                   
                                                         2716 ± 163    
Carbide Layer                                                             
Erosion Resistance                                                        
Breakthrough Time, secs                                                   
               125   125   --     65    54    65    92    73              
Calculated Erosion Rate,                                                  
               317   345   --    254   221   280   307   250              
secs/mil                                                                  
Weight Loss in --    --    --    --    --    --    --    --               
30 seconds, g                                                             
__________________________________________________________________________
 
    
     This example showed that the erosion resistance of W+W 2  C coating with a W interlayer was much better than that of W+W 3  C and W+W 2  C+W 3  C coatings with a W interlayer. This is an unexpected and significant finding. 
     EXAMPLE 7B 
     In this example, several AM-350, Ti/6Al/4V and IN-718 specimens were coated with W followed by W+W 2  C coating using conditions summarized in Table 6. All the specimens were coated with ˜6 μm and ˜15 μm W and W+W 2  C layers, respectively. The coating consisted of columnar W coating and non-columnar W+W 2  C coating at the top of W. The thickness ratio was ˜0.4. The coating was smooth. However, a few fine cracks were noted in the coating, as shown in the FIG. 12. The erosion resistance of the coating shown in Table 7 was similar to that noted in Example 7A. The weight loss during erosion test was determined and presented in Table 7. The weight loss was slightly higher in this example than that observed with W+W 2  C+W 3  C coating with W interlayer. This could be related to the fact the cracks in W+W 2  C coatings were slightly wider than those noted in W+W 2  C+W 3  C coating. 
     Examples 7A and 7B clearly showed that a tungsten to tungsten/tungsten carbide thickness ratio of ˜0.35 is sufficient enough to give a maximum erosion resistance. 
     EXAMPLE 7C 
     The CVD run described in Example 7B was repeated to provide slightly higher thickness ratio. The thickness of W+W 2  C layer varied from 10 μm to 11 μm on various substrates, and the ratio varied from 0.44 to 0.50. The coating on AM-350 and IN-718 was crack-free; whereas, a few fine cracks were observed on Ti/6Al/4V. This indicated that crack-free coating on AM-350 and IN-718 can be obtained by providing ˜5 μm thick W and ˜10 to 11 μm thick W+W 2  C layers. These thickness values, however, were not good enough to give crack-free coating on Ti/6Al/4V. The erosion resistence of the coating was very good as shown in Table 7. 
     EXAMPLE 7D 
     The CVD run described in Example 7C was repeated to provide slightly thicker W+W 2  C layer while maintaining thickness ratio. Surprisingly, the coating on AM-350 and Ti/6Al/4V cracked. This information suggested that the thickness of W+W 2  C layer was important to prevent cracks in the coating. The erosion resistance of the coating was good, as shown in Table 7. 
     EXAMPLE 7E 
     The CVD run described in Example 7D was repeated to provide slightly thinner W+W 2  C layer and slightly higher thickness ratio. The coating on AM-350, Ti/6Al/4V and IN-718 was absolutely crack-free (see FIG. 13), certifying the importance of the thickness of W+W 2  C. The unetched and etched cross-sectional views of the composite coatings are shown in the FIGS. 3 and 4. The etched cross-section presented in the FIG. 4 showed columnar growth of the tungsten interlayer and the non-columnar growth of the tungsten/tungsten carbide layer. The etched cross-section of the coating also demonstrated the absence of any cracks in the coating. The erosion resistance of the coating was good, as shown in Table 7. 
     EXAMPLES 7F AND 7G 
     The CVD run described in Example 7E was repeated in these examples to verify the concept of crack-free coating. The coatings obtained in these runs were absolutely crack-free. They also had good erosion resistance, as shown in Table 7. 
     EXAMPLE 7H 
     In this example, the thickness of W and W+W 2  C layers was slightly increased on AM-350 stainless steel to determine its effect on cracks. The thickness ratio obtained was ˜0.6. The coating on AM-350 cracked and that on Ti/6Al/4V did not. The coating on AM-350 cracked probably because of the thicker W+W 2  C layer. This information further confirmed the statement made earlier that the thickness of W+W 2  C layer played an important role in obtaining crack-free coating. Erosion resistance of the coating was good, as shown in Table 7. 
     EXAMPLE 7I 
     To further demonstrate the effect of W+W 2  C coating thickness on cracks, a CVD run was carried out to obtain a thicker W+W 2  C coating and a higher thickness ratio. Despite higher thickness ratio the coating on all the specimens cracked. This example, therefore, confirmed the importance of the thickness of W+W 2  C layer for preventing cracks in the coating. The erosion resistance of the coating was good, as expected. 
     EXAMPLE 7J 
     To further demonstrate the effect of W+W 2  C coating thickness on cracks, one more CVD run was conducted to obtain thicker W+W 2  C coating and higher thickness ratio. Once again, coating cracked. The erosion resistance of the coating, however, was still good. 
     EXAMPLES 7K TO 7P 
     Several CVD runs were conducted to vary thicknesses of W and W+W 2  C layers and thickness ratios. These experiments were conducted to effectively map out the crack-free and cracked coating region. The data summarized in Table 6 showed that as long as the thickness of W+W 2  C layer was maintained below ˜13.5 μm and thickness ratio maintained above 0.6 the coating obtained was absolutely crack-free. The coating in all the cases demonstrated good erosion resistance, as shown in Table 7. 
     Discussion on Tungsten Followed by W+W 2  C Coating 
     The erosion resistance of the W+W 2  C coating is either equivalent to or superior to the erosion resistance of W+W 2  C+W 3  C. The erosion resistance of W+W 2  C is independent of the thickness of the tungsten interlayer when the ratio of the W to the W+W 2  C thickness is at least 0.35. 
     Careful control of both the thickness of the tungsten/tungsten carbide coating and the ratio of the thicknesses of W to W+W 2  C is required to obtain a crack-free coating. For both AM-350 and Ti/6Al/4V, the maximum thickness for producing a crack-free coating is about 14 μm. The critical thickness ratio for a crack-free coating is about 0.4 and 0.5 for AM-350 and Ti/6Al/4V, respectively. 
     EXAMPLE 8 
     The wear performance of the uncoated stainless steel 1&#34; diameter SS-422 disc was determined using a ball-on-disc test in the presence and absence of a lubricant. The non-lubricated wear test was conducted in dry air (1% relative humidity) and saturated air (99% relative humidity). The lubricated wear test was conducted in the presence of a cutting fluid consisting of an emulsion of 20% mineral oil in water. The ball used in the test was made of 52-100 chrome steel. The ball-on-disc wear test was performed using a load of 5 Newton, ambient temperature, stationary ball on rotating disc at a speed of 10 cm/sec., and for approximately 0.3 kilometer. The wear performance was determined by measuring the combined volumetric material loss of the ball and disc. The wear rate was very high in the dry air (1% relative humidity), as shown in the Table 8. The wear rate in the saturated air (99% relative humidity) and cutting fluid, on the other hand, was considerably lower, with the rate being lowest in the presence of the lubricant. 
     EXAMPLE 9 
     In this example, a two-step coating process was used. Several SS-422 discs were heated to a temperature of bout 460° C. in the presence of flowing argon and at the reaction temperature a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min hydrogen, and 4,500 cc/min of argon was passed into the furnace over the specimens for 15 minutes to coat them with tungsten. After coating the specimens with tungsten for 15 minutes, a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min hydrogen, 300 cc/min argon and 40 cc/min of DME was passed into the furnace for 30 minutes to provide tungsten/tungsten carbide coating. A total pressure was maintained at 40 Torr during the tungsten as well as tungsten/tungsten carbide coating steps. 
     The SS-422 discs were coated with 10.4 μm thick tungsten followed by 12.4 μm thick tungsten/tungsten carbide. The tungsten/tungsten carbide top coat consisted of a mixture of W and W 3  C phases. The hardness of the top coat was approximately 2450 Vickers. The coating was smooth and had a few very fine interconnected cracks. 
     The wear performance of the coated SS-422 disc was determined using ball-on-disc test described in the Example 8 in the presence of 1% and 99% relative humidity and cutting fluid as a lubricant. The wear rate data summarized in the Table 8 showed dramatically lower wear rate in the presence of dry air (1% relative humidity) compared to the uncoated disc. The wear rate in the presence of 99% relative humidity and cutting fluid was also lower than the uncoated disc. 
     This example, therefore, shows that a composite coating is very effective in reducing the total wear rate in the presence and absence of a lubricant. 
     EXAMPLE 10 
     In this example, a two-step coating process was used again. Several SS-422 discs were heated to a temperature of about 460° C. in the presence of flowing argon and at the reaction temperature a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min hydrogen, and 4,500 cc/min of argon was passed into the furnace over the specimens for 15 minutes to coat them with tungsten. After coating the specimens with tungsten for 15 minutes, a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min hydrogen, 300 cc/min argon and 60 cc/min of DME was passed into the furnace for 40 minutes to provide tungsten/tungsten carbide coating. A total pressure was maintained at 40 Torr during the tungsten as well as tungsten/tungsten carbide coating steps. 
     The SS-422 discs were coated with 9.7 μm thick tungsten followed by 14.0 μm thick tungsten/tungsten carbide. The tungsten/tungsten carbide top coat consisted of a mixture of W, W 2  C and W 3  C phases. It had a hardness of approximately 2250 Vickers. The coating was smooth and had a few extremely fine and long cracks. 
     
                                           TABLE 8                                 
__________________________________________________________________________
       Composition of the                                                 
       Tungsten/Tungsten                                                  
                 Total Wear Rate, 10.sup.-15 m.sup.2 /N                   
Example No.                                                               
       Carbide Top Coat                                                   
                 1% Humidity                                              
                        99% Humidity                                      
                                Cutting Fluid                             
__________________________________________________________________________
Example 8                                                                 
       --        327    8.1     2.9                                       
Example 9                                                                 
       W + W.sub.3 C                                                      
                 1.9    7.7     1.0                                       
Example 10                                                                
       W + W.sub.2 C + W.sub.3 C                                          
                 1.0    4.7     0.9                                       
Example 11                                                                
       W + W.sub.2 C                                                      
                 1.40   3.6     1.0                                       
__________________________________________________________________________
 
    
     The wear performance of the coated SS-422 disc was determined using ball-on-disc test described in the Example 8 in the presence of 1% and 99% relative humidity and cutting fluid as a lubricant. The wear rate data summarized in the Table 8 showed lower wear rate in the presence of 1% and 99% relative humidity and cutting fluid compared to the uncoated disc and disc coated with tungsten followed by W+W 3  C coating. 
     This example shows that a composite coating is very effective in improving wear performance of the SS-422 disc. It also shows that the wear performance of the composite coating can be improved by adjusting the composition of the tungsten/tungsten carbide top coat. 
     EXAMPLE 11 
     In this example, a two-step coating process was used again. Several SS-422 discs were heated to a temperature of about 460° C. in the presence of flowing argon and at the reaction temperature a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min hydrogen, and 4,500 cc/min of argon was passed into the furnace over the specimens for 15 minutes to coat them with tungsten. After coating the specimens with tungsten for 15 minutes, a gaseous mixture of 300 cc/min WF 6 , 3,000 cc/min hydrogen, 300 cc/min argon and 80 cc/min of DME was passed into the furnace for 40 minutes to provide tungsten/tungsten carbide coating. A total pressure was maintained at 40 Torr during the tungsten as well as tungsten/tungsten carbide coating steps. 
     The SS-422 discs were coated with 10 μm thick tungsten followed by 13.0 μm thick tungsten/tungsten carbide. The tungsten/tungsten carbide top coat consisted of a mixture of W and W 2  C phases. It had a hardness of approximately 2750 Vickers. The coating was smooth and crack-free. 
     The wear performance of the coated SS-422 disc was determined using ball-on-disc test described in Example 8 in the presence of 1% and 99% relative humidity and cutting fluid as a lubricant. The wear rate data summarized in Table 8 showed the wear rate was similar to that noted with the other composite coatings (i.e., W+W 3  C and W+W 2  C+W 3  C coatings) in the presence of dry air (1% relative humidity) and cutting fluid. The wear rate, however, was lower compared to other composite coatings in the presence of 99% relative humidity. 
     This example shows that a composite coating is very effective in improving wear performance of the SS-422 disc. 
     General Discussion 
     The wear data presented in Examples 8 to 11 clearly demonstrate that a composite tungsten followed by tungsten/tungsten carbide coating can be used to significantly reduce the abrasive wear rate and concomitantly increase the life of the stainless steel material in the dry, humid and lubricated environments. The data presented in Examples 1 to 4 show that the composite coating is very effective in reducing the erosive wear rate of the ferrous and non-ferrous alloys. Additionally, Examples 1 to 4 show that an interlayer of tungsten is required to improve the performance of the tungsten/tungsten carbide coating. This is an unexpected finding. 
     The relationship between the ratio of the thickness of the tungsten interlayer to the thickness of the tungsten-carbon alloy (tungsten/tungsten carbide coating) is elaborated further in the FIG. 14. It shows that for W+W 3  C top coat the erosion resistance measured as secs/mil increases with increasing the thickness ratio. A thickness ratio greater than 0.3 is required to significantly increase the erosion resistance of the W+W 3  C coating system. Furthermore, a thickness ratio of about 0.6 is required to obtain optimum erosion resistance of the W+W 3  C coating system. FIG. 14 also shows that a thickness ratio of greater than 0.3 is required for significantly increasing the erosion resistance of the W+W 2  C+W 3  C coating system. It also shows that a thickness ratio of about 0.35 yields optimum erosion performance both for the W+W 2  C+W 3  C and W+W 2  C coating systems. This thickness ratio to obtain optimum erosion resistance is considerably lower for the W+W 2  C+W 3  C and W+W 2  C coating systems than the W+W 3  C coating system. 
     The relationship between the thickness of the tungsten-carbon alloy and the ratio of the thickness of the tungsten interlayer to the thickness of the tungsten-carbon alloy (tungsten/tungsten carbide coating) is presented in FIG. 15. It shows a very narrow region for obtaining a crack-free W+W 3  C coating system. A thin W+W 3  C layer is required to achieve a crack-free coating. Compared to the W+W 3  C coating system, the W+W 2  C+W 3  C and W+W 2  C coating systems provide a wider crack-free coating region. 
     It is worth noting that a thicker crack-free tungsten/tungsten carbide coating can be obtained for W+W 2  C top coat than is possible for W+W 2  C+W 3  C or for W+W 3  C. In addition, the thickness of the tungsten interlayer which is required to achieve a crack-free coating is significantly lower for W+W 2  C than for W+W 2  C+W 3  C or W+W 3  C. By reducing the thickness of the W interlayer, the overall thickness of the composite coating which is required to provide a specified erosive wear life or abrasive wear life can be minimized by going progressively from W+W 3  C to W+W 2  C+W 3  C and to W+W 2  C composite coating system. 
     One particularly important use of the composite coating system according to the present invention is to provide highly erosive and abrasive wear resistant coatings on ferrous, non-ferrous and titanium alloy compressor blades for gas turbines and jet engines.